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Christchurch earthquake triggers design code review

New Zealand seismic and building design codes are being subjected to a major review following February’s devastating earthquake in Christchurch.

Changes already underway

Some changes are already being implemented.

Following the Christchurch quake, officials believe there is a heightened risk of seismic activity in the region.
The Institution of Professional Engineers New Zealand (Ipenz) is considering whether to recommend that structures are designed to withstand one in 5,000 year events instead of one in 475 year events.

“Increased return periods have certainly been discussed,” Ipenz practice manager Cameron Smart told NCE.

“Typically you design a structure to withstand an earthquake that has a 10% chance of occurring over 50 years,” added Arup seismic specialist Ziggy Lubkowski. This equates roughly to a one in 475 year event.

“Increased return periods have certainly been discussed”

Ipenz practice manager Cameron Smart

Smart added that retrofit regulations could also face changes. Currently New Zealand law requires buildings designated as “earthquake prone” to be upgraded to at least one third of current building codes − or resistant to at least a third of the loads demanded by the earthquake design code.

But the New Zealand Society for Earthquake Engineering has been arguing that this requirement be revised upwards to two thirds of earthquake code design loadings.

Smart added that it could become a problem for heritage buildings which are “asset rich but cash poor” and some architects are resisting such a change.

Some changes to the Christchurch region’s building codes have already been implemented. New Zealand’s Department of Building and Housing has strengthened the definition of “good ground” and all concrete floor foundations in the high risk region must now be tied and reinforced.

Costly changes

But this will be costly as official estimates put the price at up to NZ$9,000 (£4,400) per property to implement.
In addition, the department said that foundations on ground that is prone either to liquefaction or lateral spread should be specially designed.

Meanwhile, the clean-up operation in Christchurch continues.

New Zealand officials have set up an agency called the Canterbury Earthquake Recovery Authority (Cera) to lead reconstruction efforts. It has a budget of £2.68bn over six years to help the rebuild.

A spokesman from consultant Opus, which is working on the reconstruction programme, said “the immediate focus has been to get citizens prepared for winter”.

Christchurch details

February’s magnitude 6.3 earthquake caused huge ground shaking in the city of Christchurch and surrounding areas.

Christchurch experienced massive peak ground accelerations of over 2.2g − where the vertical forces exerted by the quake are 2.2 times gravity at their peak.

Scientists say the close proximity of the epicentre to the surface and the “trampoline” interaction of the geological layers under the city combined to produce dramatic ground motions.

New Zealand’s unique location between two tectonic plates makes it especially prone to seismic activity.

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